Coordination-triggered redox activity of early and late lanthanide calix[4]arene complexes.

The methylation of p-tert-butylcalix[4]arene in the distal 1,3-phenolic sites provides H2L = {p-tert-butylcalix[4](OMe)2(OH)2arene}. This unit acts as a rigid coordinating ligand to early and late lanthanide metal ions, enabling the construction of two families of mononuclear compounds featuring (N(nBu)4)[LnIIIL(acac)2]·CH3CN (Ln = Pr (1), Nd (2), Ho (3), and Er (4)) and (N(nBu)4)2[LnIIIL{Mo5O13(OMe)4(NO)}]·CH2Cl2 (Ln = Nd (5) and Er (6)). The metal ions adopt distorted bicapped trigonal prismatic coordination environments, resulting in slow relaxation of the magnetization for 4. These compounds exhibit reversible redox waves at positive potentials, centered within the calix[4]arene ligand, representing a new type of calix[n]arene-based electrochemical activity induced by coordination to the metal centers.

Organoarsonates Enable Single-Site Condensation of Hexalacunary {P2W12} Polyoxotungstates.

A systematic study of the condensation reactions of arylarsonic-functionalized [α-P2W12O48]14- units in acidic aqueous media identified that the specific presence of an amino group in the ortho position of the phenyl rings induces a dimerization process that allowed isolation of discrete dimeric polyanions [(o-H2N-C6H4-AsO3)4P4W24O85]14- (1) with an unprecedented polyoxometalate skeleton characterized by two seminal {P2W12} groups joined via a single W-O-W bridge. At the same time, addition of divalent transition metal ions (MnII, CoII, and NiII) in the reaction mixture directed a condensation process on a completely different pathway resulting in one-dimensional (1D) coordination polymers based on V-shaped [{M(H2O)4}P4W24O92(C6H6AsNO)2]14- polyanions (M = MnII (2), CoII (3), and NiII (4)). All polyanions were isolated as hydrated mixed potassium/dimethylammonium salts and thoroughly characterized in the solid state. 31P NMR studies showed that the discrete cluster 1 is comparatively stable in 1 M LiCl aqueous solution and thus represents a potential precursor for subsequent reactions.

Tris-decorated multi-iron polyoxotungstates.

Solution-stable tris(hydroxymethyl)aminomethane-functionalized FeIII-containing polyoxotungstates exhibit an unusual anchoring mode of triol moieties, with one -NH2 and one -CH2OH group remaining accessible for post-functionalization or chemisorption. The redox-active title compounds have been isolated under unusually mild reaction conditions and characterized in the solid state and in aqueous solutions.

Controlled Hydrolysis of Phosphate Esters: A Route to Calixarene-Supported Rare-Earth Clusters.

Phosphate ester bonds are widely present in nature (e. g. DNA/RNA) and can be extremely stable against hydrolysis without the help of catalysts. Previously, we showed how the combination of phosphoryl and calix[4]arene moieties in the same organic framework (LPO ) allows isolation of single lanthanide (Ln) metal ions as [LnIII (LPO )2 ](O3 SCF3 )3 . Here we report how by controlling the reaction conditions a new hydrolyzed phosphoryl-calix[4]arene ligand (H3 LHPO ) is formed as a result of LnIII -mediated P-OEt bond cleavage in three out of the eight possible sites in LPO . The chelating nature of H3 LHPO traps the LnIII species in the form of [LnIII (LHPO )((EtO)2 P(O)OH)]2 dimers (Ln=La, Dy, Tb, Gd), where the Dy derivative shows slow magnetization relaxation. The strategy presented herein could be extended to access a broader library of hydrolyzed platforms (Hx LHPO ; x=1-8) that may represent mimics of nuclease enzymes.

Hybrid lanthanide double-deckers based on calixarene and polyoxometalate units.

Complementarity of calixarene (H2L) and polyoxometalate ligands results in the first organic-inorganic [MIIIL{Mo5O13(OMe)4(NO)}]2- (M = Y, Gd and Dy) hybrid, directing the formation of a distorted square-antiprismatic MO8 ligand field and resulting in slow relaxation of the magnetisation for the Dy derivative.

Trigonal Prismatic Coordination of Discrete Rare Earth Ions, Enforced by the Polyoxotungstate [P4 W27 O99 (H2 O)]16.

A family of solution-stable polyanions [Na⊂{LnIII (H2 O)}{WVI O(H2 O)}PV 4 WVI 26 O98 ]12- (Ln=Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu and Y) represent the first examples of polyoxometalates comprising a single lanthanide(III) or yttrium(III) ion in a rare trigonal prismatic O6 environment. Their synthesis exploits the reactivity of the organophosphonate-functionalized precursor [P4 W24 O92 (C6 H5 PV O)2 ]16- with heterometal ions and yields hydrated potassium or mixed lithium/potassium salts of composition Kx Lny H12-x-y [Na⊂{Ln(H2 O)}{WO(H2 O)}P4 W26 O98 ]⋅nH2 O⋅mLiCl (x=8.5-11; y=0-2; n=24-34; m=0-1.5). The Dy, Ho, Er and Yb derivatives are characterized by slow magnetization relaxation.

Expansion of Zirconium Oxide Clusters by 3d/4f Ions.

Two series of charge-neutral coordination clusters featuring quasi-isostructural metal oxide cores, isolated as [Zr6Fe2Ln2O8(ib)14(bda)2(NO3)2]·xMeCN (Ln = La (1), Ce (2), Pr (3), and Nd (4); ib- = isobutyrate; H2bda = N-butyldiethanolamine) and [Zr6Fe2Ln2O8(ib)14(mda)2(NO3)2]·xMeCN (Ln = La (5), Ce (6), Pr (7), and Nd (8); H2mda = N-methyldiethanolamine), were obtained via one-pot reactions of [Fe3O(ib)6(H2O)3]NO3 as a critical precursor, Ln(NO3)3·6H2O (Ln = La, Ce, Pr, and Nd), the respective aminoalcohol, and [Zr6O4(OH)4(ib)12(H2O)]·3Hib in an acetonitrile solution. The coordination clusters in 1-8 feature {Zr6O8} cores that are structurally expanded by two 4f (Ln3+) and two 3d (Fe3+) metal ions, each individually coordinated to one of the eight oxide centers of {Zr6O8}, producing a metal skeleton where the 3d/4f positions cap four of the triangular faces of the central Zr6 octahedron. The coordination clusters differ in the chosen aminoalcohol coligands, N-butyldiethanolamine or N-methyldiethanolamine, which lead to a different isobutyrate coordination pattern in the two series, while the {Fe2Ln2Zr6O8} core structure remains virtually unaffected. All eight coordination clusters are obtained in moderate to good yields of 29-66% after only several days. Complexes 1-8 are stable against air and moisture; they are also surprisingly thermally stable up to 280 °C in air and in nitrogen atmosphere, and they represent the first reported examples of 3d/4f-functionalized zirconium oxide clusters.

Phosphorylated-calix[4]arene double-deckers of single rare earth metal ions.

Combination of phosphoryl and calix[4]arene moieties in the same organic framework (LPO) directs the formation of homoleptic double-decker complexes [LnIII(LPO)2](OTf)3 for Ln = Tb and Dy, with the latter displaying slow relaxation of the magnetisation.

Exploiting complementary ligands for the construction of square antiprismatic monometallic lanthanide SMMs.

The methylation of p-tert-butylcalix[4]arene in the distal 1,3-phenolic sites provides the ligand H2L = {p-tert-butylcalix[4](OMe)2(OH)2arene} that enables construction of heteroleptic mononuclear lanthanide complexes. The reaction of (N(nBu)4)(acac) (Hacac = acetylacetone), MIIICl3 and H2L under Schlenk conditions results in the formation of a family of (N(nBu)4)[MIIIL(acac)2] complexes where M = Y (1), Gd (2), Tb (3) and Dy (4). The metal ions are eight-coordinate in distorted square-antiprismatic coordination geometries, resulting in slow relaxation of the magnetisation for the Tb derivative.

Insertion of VIV Ions into the Polyoxotungstate Archetype {As4W40}.

A new polyoxometalate compound consisting of the 39-tungsto-4-arsenate(III) unit with four incorporated VIV ions, isolated as (NH4)22[(VIVO)2(VIVO(H2O))(AsIIIWVI9O33)2(AsIIIWVI8.5VIV0.5(OH)O32)2(WVIO2)4]·48H2O (NH4-As4W39(V4)), was synthesized and fully characterized. SQUID magnetometry shows three weakly coupled VIV centers with an antiferromagnetic exchange interaction and one isolated VIV ion as a spin-1/2 Curie paramagnet. UV-vis spectroscopy indicates that the As4W39(V4) structure remains intact in aqueous solution for at least 24 h. To enable the deposition of As4W39(V4) from solution on gold surfaces, its trihexyltetradecylphosphonium salt, THTDP-As4W39(V4), was prepared. The IR spectra of both congeners reveal the structural identity of As4W39(V4) independent of the countercations. The X-ray absorption near-edge structure data confirm the presence of VIV centers in a distorted square-pyramidal coordination geometry in NH4-As4W39(V4) and THTDP-As4W39(V4). X-ray photoelectron spectroscopy of the latter, deposited on Au(111), shows that the 4 V and 35 W centers preserve their IV+ and VI+ oxidation states, while the remaining 4 W ions are reduced to IV+.

Polyoxotungstate Archetype {P4 W27 } and its 3d Derivatives.

The propensity of the new, phenylphosphonate-stabilized polyoxotungstate [(C6 H5 PV O)2 P4 W24 O92 ]16- to act as a precursor for new 3d metal-functionalized polyanions has been investigated. Reactions with MnII and CuII induce the formation of the previously unknown polyoxotungstate archetype {P4 W27 }, isolated as salts of the polyanions [Na⊂{MnII (H2 O)}{WO(H2 O)}P4 W26 O98 ]13- (1) and [K⊂{CuII (H2 O)}{W(OH)(H2 O)}P4 W27 O99 ]14- (2), which were characterized in the solid state (single-crystal X-ray diffraction, elemental and TG analyses, IR spectroscopy, SQUID magnetometry) and in aqueous solution (UV/Vis spectroscopy, cyclic voltammetry).

Polyoxometalates with separate lacuna sites.

Bi-functionalizion of Wells-Dawson-type polyoxotungstates with exo-coordinated rare earth metal ions (M = DyIII, YIII) and phenylphosphonate induces the formation of a separate second lacunary site in the pre-functionalized [α2-P2W17O61(PhPO)2]6- precursor. The resulting [{(H2O)xM2(α2,α2'-P2W16O60)(PhPO)2}2]8- dimer clusters are stable in solution, exhibit interesting electrochemical behavior, and, for the DyIII derivative, single ion-magnet characteristics.

Intramolecular crossover from unconventional diamagnetism to paramagnetism of palladium ions probed by soft X-ray magnetic circular dichroism.

The case of palladium(II) ions in molecular polyoxopalladates highlights the importance of accounting not only for nearest neighbour atoms or ions in order to understand, model or predict magnetic characteristics. Here, using site-specific soft X-ray magnetic circular dichroism (XMCD), the effects of different bond lengths, delocalization of 4d electrons, and 4d spin-orbit coupling on the electronic and magnetic properties are investigated and three different states identified: Conventional diamagnetism in a square-planar O4 coordination environment, paramagnetism caused by four additional out-of-plane oxygen anions, and an unusual diamagnetic state in the diamagnetic/paramagnetic crossover region modified by significant mixing of states and facilitated by the substantial 4d spin-orbit coupling. The two diamagnetic states can be distinguished by characteristic XMCD fine structures, thereby overcoming the common limitation of XMCD to ferro-/ferrimagnetic and paramagnetic materials in external magnetic fields. The qualitative interpretation of the results is corroborated by simulations based on charge transfer multiplet calculations and density functional theory results.

Conductive Self-Assembled Monolayers of Paramagnetic {CoII Co 4 III } and { Co 4 II Co 2 III } Coordination Clusters on Gold Surfaces.

Two polynuclear cobalt(II,III) complexes, [Co5(N3)4(N-n-bda)4(bza·SMe)2] (1) and [Co6(N3)4(N-n-bda)2(bza·SMe)5(MeOH)4]Cl (2), where Hbza·SMe = 4-(methylthio)benzoic acid and N-n-H2bda = N-n-butyldiethanolamine, were synthesized and fully characterized by various techniques. Compound 1 exhibits an unusual, approximately C 2-symmetric {CoII Co 4 III } core of two isosceles Co3 triangles with perpendicularly oriented planes, sharing a central, high-spin CoII ion residing in a distorted tetrahedral coordination environment. This central CoII ion is connected to four outer, octahedrally coordinated low-spin CoIII ions via oxo bridges. Compound 2 comprises a semi-circular { Co 4 II Co 2 III } motif of four non-interacting high-spin CoII and two low-spin CoIII centers in octahedral coordination environments. Self-assembled monolayers (SAMs) of 1 and 2 were physisorbed on template-stripped gold surfaces contacted by an eutectic gallium-indium (EGaIn) tip. The acquired current density-voltage (I-V) data revealed that the cobalt-based SAMs are more electrically robust than those of the previously reported dinuclear {CuIILnIII} complexes with Ln = Gd, Tb, Dy, or Y (Schmitz et al., 2018a). In addition, between 170 and 220°C, the neutral, mixed-valence compound 1 undergoes a redox modification, yielding a {Co5}-based coordination cluster (1-A) with five non-interacting, high-spin octahedral CoII centers as indicated by SQUID magnetometry analysis in combination with X-ray photoelectron spectroscopy and infrared spectroscopy. Solvothermal treatment of 1 results in a high-nuclearity coordination cluster, [Co10(N3)2(N-n-bda)6(bza·SMe)6] (3), containing 10 virtually non-interacting high-spin CoII centers.

Blurring the line between addenda and heteroatoms in a giant polyoxotungstotellurite.

The 3.5 nm-sized polyanion [H4TeWO372]80-, exhibiting an exceptionally high Te : W ratio, expands the structural role of TeIV ions from classical heterogroups within polyoxotungstate subunits to addenda ions. Investigations on the self-assembly conditions, proceeding in a simple one-pot reaction, point to the pH as one critical factor for circumventing this limitation.

Sequential Isomerization of a Macrocyclic Polyoxometalate Archetype.

Controlled isomerization of individual {α-P2W12O48} polyoxotungstate building blocks under the constricted conditions of the macrocyclic [P8W48O184]40- archetype ({P8W48}) is linked to site-specific CuII coordination. The derivatives [αγαγ-P8W48O184{Cu(H2O)}2]36- (1), [γγγγ-P8W48O184{Cu(H2O)0.5}4]32- (2), and [αγγγ-P8W48O184{Cu(H2O)}3]34- (3) feature the {αγαγ-P8W48} and the hitherto unknown {γγγγ-P8W48} and {αγγγ-P8W48} isomers based on {α-P2W12} and/or CuII-stabilized {γ-P2W12} units and form from the reactions of the classical {P8W48} (={αααα-P8W48}) and CuCl2 in sodium acetate medium (pH 5.2). All products were thoroughly characterized in both the solid state and aqueous solutions, including electrocatalysis assessments.

Host-Guest-Induced Environment Tuning of 3d Ions in a Polyoxopalladate Matrix.

A series of unprecedented supramolecular associates of phenylarsonate-capped {MII PdII 12 O8 }-type (M=Co, Ni and Zn) polyoxopalladates with α-cyclodextrins (α-CD) was obtained and characterized in the solid state (single-crystal X-ray diffraction (XRD), FT-IR spectroscopy, elemental and thermogravimetric (TGA) analyses), in aqueous solution (1 H and 13 C NMR) and in the gas phase (ESI-MS). The non-covalent host-guest interactions between the organopolyoxoanions and α-CD rings alter the O8 coordination environment of a 3d transition metal ion (MII ) situated at the center of a cuboid polyoxododecapalladate shell. This synthetically controlled "chemical pressure" effectively induces axial distortion of the otherwise cubic polyoxopalladate environment between two trans-positioned α-CD moieties. Its effect on the magnetic properties and the electronic structure of the CoII derivative was assessed in a combined SQUID magnetometry, EPR, X-ray magnetic circular/linear dichroism (XMCD/XMLD), and X-ray absorption near-edge structure (XANES) spectroscopy study.

Element-Selective Molecular Charge Transport Characteristics of Binuclear Copper(II)-Lanthanide(III) Complexes.

A series of isostructural dinuclear 3d-4f complexes, isolated as [CuLn(L·SMe)2(OOCMe)2(NO3)]· xMeOH (Ln = Gd 1, Tb 2, Dy 3, and Y 4; x = 0.75-1) and comprising one acetate and two thioether-Schiff base (L·SMe-) bridging ligands based on 4-(methylthio)aniline and 2-hydroxy-3-methoxybenzaldehyde (HL·SMe = C15H15NO2S), was synthesized and fully characterized. The magnetic properties of the charge-neutral {CuLn} complexes are dominated by ferromagnetic CuII-LnIII exchange interactions. Large-area electron transport studies reveal that the average conductivity of robust, self-assembled {CuLn} monolayers on a gold substrate is significantly lower than that of common alkanethiolates. Theoretical calculations of transmission spectra of individual complexes 1 and 4 embedded between two metallic electrodes show that the molecular current-voltage ( I- V) characteristics are strongly influenced by electron transport through the Cu centers and thus fully independent of the lanthanide ion, in excellent agreement with the experimental I- V data for 1-4. The ß-polarized transmission indicated by calculations of 1 and 4 points out their potential as spin filters. In addition, the reactivity of the title compound 1 with CuII in a square-pyramidal coordination environment toward methanolate and azide was examined, resulting in the formation of a linear trinuclear complex, [Cu2Na(L·SMe)4]NO3·3MeOH (5), characterized by antiferromagnetic exchange interactions between the two copper ions.

The homometallic polyoxotungstate archetype {P4W24}.

Functionalization with terminal organophosphonate and -arsonate groups allows for crystallization and structural characterization of elusive homometallic phosphotungstates of the {PW} archetype. 31P NMR measurements reveal moderate stability in aqueous solution for the [(PhPO)2P4W24O92]16- derivative, enabling consecutive reactions as exemplified by the formation of a CoII-{P4W24}-based coordination polymer.

Organoarsonate Functionalization of Heteropolyoxotungstates.

Functionalization of the {P8W48} polyoxotungstate (POT) archetype with aromatic organoarsonates results in the first homometallic {P8W48} derivatives, with the general formula [(RAsVO)4PV8WVI48O184]32- [R = C6H5 (1) or p-(H2N)C6H4 (2)]. Short As-O bonds here induce unusual bending of the otherwise rigid {P8W48} macrocycle, breaking its D4h symmetry. The obtained species also represent the first lacunary POTs functionalized with organoarsonates and can potentially act as polyoxometalate precursors themselves. We elaborate solution stability in different aqueous media using 1H and 31P NMR spectroscopy and possible pathways for subsequent transformations in aqueous solutions of the functionalized polyanions. Recrystallization of the K+/Li+/dimethylammonium salt of 2 from 4 M LiCl solution yielded a further functionalized POT, [(H3NC6H4AsO)3P8W48O184Hx{WO2(H2O)2}0.4](30.2-x)- (3), revealing dissociation of the organoarsonate fragments in slightly acidic aqueous solutions followed by their rearrangement within the inner POT cavity.